measuring 101

[corvettekid1973]

Someone please school me on this. I have two 17x8.5 vette wheels with a 56mm offset. I just measured the backspacing and it looks like 6 3/4 inches. But when I add that to the 56mm/2.2" offset it comes out over 8.5 inches. is offset not from the mounting surface to the outside of the rim? help me cuz I be stoopid lol

[65_LS1_T56]

Offset (56mm) is from the wheel centerline, or halfway accross it's width, to the WMS, or wheel mounting surface. Backspace is from the external face of the wheel (on the inboard side towards the vehicle centerline) to the WMS. So and 8.5" rim is actually about 9.5" overall width (approximately, measure yours). If you take 56mm or 2.20", the backspace should be about 6.95" on that wheel (9.5/2 + 2.2"). Of course the actual outside rim width affects this calculation.

Hope this helps.

[corvettekid1973]

OK I really appreciate your help. I think I got what you are saying. I understand offset now, but I measured my backspacing from the outside on the back of the rim, with my straight edge in between the small lip that sticks out as in my 1st pic, p.s. took pic without straight edge to make it easier. But to get a accurate measurement of the rim width it has to be measured inside the rim with the tire off as in the second pic right? The thickness of material changes measurements. Sorry I posted pics in the wrong order

[alky]

http://www.rimsntires.com/specs.jsp

This site is great

[Forgeline Wheels]

You might be interested in this quick little measurement explanation video from Dave.
http://youtu.be/2K1lUlv9ZOA
"Forgeline's David Schardt explains the difference between backspace and offset and then demonstrates how to measure backspace and calculate offset."

[Daren]

I like this best:

https://www.rsracing.com/tech-wheel.html

[corvettekid1973]

Thanks guys. I'm in the process of putting 17x11 and 17x8.5's on my car and I'm figuring out what size spacers to run. I know spacers aren't the best option but I'm doing this on a budget until I can buy the proper fitting wheels